Delphi Basics : $RangeChecks command

DelphiBasics

$RangeChecks
Compiler Directive

Determines whether Delphi checks array bounds

{$RangeChecks Off}

1
2	{$RangeChecks On}

Description

The $RangeChecks compiler directive determines whether Delphi should add code for array bounds checking.

This is set Off by default, meaning that a bad array access will pass unnoticed, revealing itself in a difficult to debug part of the code.

It is recommended to switch on $RangeChecks in order to detect array bound problems. This will result in the raising of an exception, allowing code testing to correctlt identify the point of failure.

Notes

$RangeChecks is equivalent to $R.

It can and should only be set once in your code.

The default value is $RangeChecks Off.

Related commands

$R		Determines whether Delphi checks array bounds
$Resource		Defines a resource file to be included in the application linking

Download this web site as a Windows program.

Example code : Trapping array bound problems

var
  myArray : array[1..5] of string;
  i : Integer;

begin
  // Set range checking on
  {$RangeChecks On}

  // Start array assignment from 0 - normally OK, but our
  // array starts at 1.
  for i := 0 to 5 do
  begin
    myArray[i] := 'Element '+IntToStr(i);
    ShowMessage('myArray['+IntToStr(i)+'] = '+myArray[i]);
  end;
end;

Show full unit code

Delphi throws the ERangeError exception

Example code : Ignoring array bounds problems

var
  myArray : array[1..5] of string;
  i : Integer;

begin
  // Set range checking off
  {$RangeChecks Off}

  // Start array assignment from 0 - normally OK, but our
  // array starts at 1.
  for i := 0 to 5 do
  begin
    myArray[i] := 'Element '+IntToStr(i);
    ShowMessage('myArray['+IntToStr(i)+'] = '+myArray[i]);
  end;
end;

Show full unit code

  myArray[0] = Element 0
  myArray[1] = Element 1
  myArray[2] = Element 2
  myArray[3] = Element 3
  myArray[4] = Element 4
  myArray[5] = Element 5